SMTP:use SMTP relaying. Use this setting when you are running LimeSurvey on a host that is not your mail server.
Make sure that you are using your SMTP configured e-mail (Global settings -> E-mail settings) at the survey settings (At the moment you create a survey or after you create it Edit Survey Settings -> General) if you use SMTP, otherwise there might be a chance that the following error is displayed: SMTP -> ERROR: RCPT not accepted from server: 553 5.7.1 : Sender address rejected: not owned by user.